Empirical geochemistry points to continents’ role in mantle dynamics

Major-element chemistry of basalts provides proxies for key parameters involved in magmatism. Sodium content, normalized to an MgO content of 8%, relates to the degree of mantle melting, and similarly normalized iron content helps assess the depth of melt production. Such proxies help establish potential mantle temperatures – the temperature of magma that would erupt after rising adiabatically from different mantle depths. Low Na8.0 suggests high potential temperature in a magma’s source.

Vast repositories of basalt chemistry relate to every conceivable setting of magmatism, so Na8.0 and Fe8.0 numbers are useful in testing various hypotheses. One of these is that slabs of continental lithosphere affect mantle convection, by forming insulating “lids” that control surface heat flow. Eric Humler and Jean Besse, of the Université Denis Diderot in Paris, focus on the relationship between mantle potential temperature beneath ocean-ridge systems and their distance to passive continental margins (Humler, E. & Besse, J. 2002. A correlation between mid-ocean ridge basalt chemistry and distance to continents. Nature, v. 419, p, 607-609). Leaving out the complicating factors of continental margins that involve subduction and ridges affected by hot spots, they found that recent ridge basalts show higher potential temperatures when the ridge is close to continental lithosphere than for more distant ridges. This suggests that the mantle cools away from continents by between 0.05 to 0.1°C per kilometre. This matches the well-known increase in depth to ridges as they become further from continents. Rather than being inert passengers on modern plates, continents do play a role in the mantle’s thermal structure.

The scope for synopsis of geochemical data is boosted by wider availability of existing data. How tedious it used to be, trawling paper journals for tables of analyses with which to compare ones own. It is still quite a task, but there is light on the horizon, because geochemists at the University of Mainz in Germany have made their compilations for ocean-island volcanic rocks and those from large igneous provinces (flood basalts) available on the web as the initial input to the GEOROC (Geochemistry of Rocks of the Oceans and Continents) database (http://georoc.mpch-mainz.gwdg.de ). A similar database for ocean-floor basalts is PETDB at Columbia University in the USA (http://petdb.ldeo.columbia.edu/petdb/). Between them, the two web sites amass over 200 thousand analyses of major- and trace-elements, and isotopes, enough for even the most ardent user of MS Excel!

Evidence for the degree and longevity of geochemical heterogeneities in the mantle has largely stemmed from studies of basalts derived by mantle melting. The great diversity of melting and fractionation processes involved in their genesis obviously complicates assessment of whether or not the mantle is a mixture of several chemical domains, even though it is suspected. Indeed it is only to be expected as a result of 4.5 billion years of mantle melting events and recycling of surface materials that find their way into subduction zones, unless, that is, long-term convection is an efficient means of mixing. A novel approach by a team from Stanford University, the University of Copenhagen and the US Geological Survey uses a combination of the rhenium-osmium radioactive decay scheme and the tendency for Re to enter melts, while Os is highly compatible to address this long-standing conundrum (Meibom, A. et al. 2002. Re-Os isotopic evidence for long-lived heterogeneity and equilibration processes in the Earth’s upper mantle. Nature, v. 419, p. 705-708). The novelty lies in their use of detrital grains of platinoids in alluvium derived from the many ultramafic masses in the western USA, rather than individual basalts or peridotites themselves.

Measurements of 187Os/188Os in the grains span a wide range from extremely unradiogenic values to those signifying a high component of radiogenic 187Os. The data occupy a bell-shaped (Gaussian) frequency distribution. While that probably reflects equilibration of old, unradiogenic material with radiogenic Os in melts derived from the mantle ultramafic rocks, and the destruction of any age information, it does point to mantle dotted with patches with different origins.

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